HVAC (Heating, Ventilation, & Air Conditioning) systems are used to meet occupant comfort and ventilation needs within a building space. Typically this involves the conditioning of air circulated to and from the space served via an air handler of some form, e.g., fans and blowers. Conditioning the air can include any combination of heating, cooling, filtering, humidifying, & dehumidifying air in a defined building space. Additionally, most HVAC systems have provision for supplying minimum amounts of fresh outside air to insure proper ventilation for human occupants.
HVAC systems include constant volume systems and variable volume systems. Variable volume systems tend to be larger in capacity and generally more sophisticated in terms of control features. Constant volume rooftop packaged units and split systems equipped with economizers are much more common than variable volume systems. Enhanced features are rare in these systems due to cost considerations. Constant volume systems, as their name suggest, deliver a constant volume of air to the building. Moreover, constant volume systems typically serve a single zone or segregated space within a building.
The operation of these systems typically involves a room thermostat controlling the heating, cooling, and ventilation modes based on whether the space is occupied and the programmed heating and cooling set points. Whenever a non-residential space is open for business or has workers in the facility, the space is considered to be in the “Occupied Mode.” In a constant volume system, the HVAC system's fan is commanded to operate at full capacity and the economizer provides minimum outside air for occupant ventilation throughout the occupied workday without regard to the temperature of the outside air. Changes in space temperature result in the thermostat sending commands to heat or cool the air being supplied to the space as necessary. The total amount of energy required to heat or mechanically cool the air is impacted by the temperature of the outside air entering the system. The colder the outside air is relative to the space, the greater the amount of energy required to heat the air. The hotter the outside air is relative to the space, the greater the amount of energy required to cool the air.
In virtually all constant volume systems the amount of outside air supplied to the space for ventilation is set for the maximum number of potential occupants anticipated in the space. For instance, a restaurant may have a dining room rated for 50 occupants. In this case, the HVAC system's minimum outside air setting will be based on the code required ventilation rate for 50 occupants. However, the dining room served by this HVAC system may only have 50 occupants at peak business hours or on rare occasions. The result is an over-ventilated space whenever there are less than 50 occupants. As stated previously, there is an energy cost associated with heating or cooling outside air. The status quo approach to occupant ventilation with these systems results in unnecessary energy usage.
Constant volume HVAC systems typically operate indoor blower motors at full capacity throughout the occupied period. The reality is that the fans do not need to operate at full capacity. Manufacturers provide a range of operation for acceptable airflow in the heating and cooling modes. A fan at full capacity typically exceeds the minimum allowable requirements. The ability to properly ventilate the space does not require the fan to be operated at 100 percent airflow. Once again these systems are not equipped with the ability to reduce fan speed and air volume in response to the true needs of space.
Sophisticated variable volume systems are able to vary the volume of air based upon the needs of the space. This can be achieved via older technologies such as inlet guide vanes or discharge dampers. Increasingly, variable volume systems rely on Variable Frequency Drives (VFD) for fan control. A VFD directly controls the speed of the fan and the air volume by reducing the motor revolutions per minute (RPM). Fan affinity laws prove that a 10% reduction in air volume or flow equals a 27% reduction in energy usage. This exponential energy dividend makes VFDs a highly valued energy efficiency tool. Once again, the cost of the VFD and the associated sensors, wiring, and installation labor has made the prospect of applying this technology to simple systems impractical. Additionally, the industry prior art has failed to identify a control strategy for applying variable volume technologies to retrofit a system designed to move a constant volume of air.
Constant volume systems are able to respond to the impact of changes in occupancy levels as they affect temperature, but they have no ability to respond to the varying ventilation needs associated with changing occupancy levels. For this to occur, these systems must have more intelligence and dynamic control capability.
Constant volume systems generally come in two distinct HVAC system types: rooftop packaged units and split systems. Rooftop packaged units are typically self-contained units mounted on a roof. Split systems typically include two sections: an indoor air handler/heating section and an outdoor compressor section connected to each other with refrigerant piping. Many of these systems are equipped with economizers. An economizer consists of mechanically-actuated outside air and return air dampers, temperature/humidity sensors, and an economizer controller. These components act together in such a way as to vary the amount of fresh outside air introduced by the HVAC system into the building space. The primary purpose of an economizer is to allow the HVAC system to utilize outside air for “free cooling” in the event that the space requires cooling and the outside air is suitable to be used as a source of cold air to cool the space. This allows the HVAC system to avoid the expense of operating the air conditioning compressor to make cold air.
Economizers are effective at lowering energy consumption if they are controlled properly and in good working order. Many studies by various utilities, energy consulting groups, and professional organizations report that 60-80% of economizers in the field are not working properly. Even properly working economizers often lack appropriate limitations on their operation when the HVAC system is operated during the unoccupied period (morning warm-up or night setback). Constant volume HVAC systems operate the economizer whenever the fan operates even though is not necessary to ventilate an unoccupied space. The most common flaw as it relates to ventilation during the occupied period is the improper positioning of the outside air damper resulting in an over-ventilated condition.
In larger, variable volume HVAC systems, one strategy to address this is through “Demand Control Ventilation” (DCV). The benefit of DCV is derived from being able to position the outside air damper to a closed or nearly closed condition unless there is a measured need for additional fresh air to the space. This is achieved by the use of an occupancy sensor. While other mechanisms may exist for calculating the occupancy level of a building, monitoring carbon dioxide levels is the most common. In such a case, a carbon dioxide (CO2) sensor is mounted in the building space or in the return air duct. Human occupants exhale carbon dioxide and an increase in the number of occupants will produce a corresponding increase in the CO2 levels. A controller is used to monitor the CO2 levels and modulate the outside air damper open as necessary to dilute the CO2 levels with fresh air. This dynamic approach to ventilation control eliminates the problem and energy expense associated with over-ventilating that comes with conventional strategies but has only rarely been applied as a retrofit measure with constant volume systems and not with ventilation fan reduction and/or heating/cooling fan reduction.
A traditional economizer in a constant volume HVAC system uses outside air for free cooling as an alternative to mechanical cooling compressor operation. The economizer controller determines the operation of the economizer by referencing the temperature and/or humidity of the outside air. When the thermostat communicates a call for cooling to the HVAC system, the economizer controller determines if the outside air is suitable for free cooling. If so, the outside air damper is modulated open and mechanical cooling is held off. The point at which this transition occurs is referred to as the “changeover point.” If the outside air is not suitable, the economizer controller keeps the outside air damper in the minimum ventilation position and commands the compressor on for mechanical cooling.
In larger, variable volume HVAC systems, an “integrated economizer” strategy is implemented. This allows the simultaneous use of the compressor for mechanical cooling and outside air economization. The use of outside air may not be suitable for meeting the total cooling load but can still work to lower the energy consumption of the system. Whereas traditional economizer logic allows either the compressor OR outside air to function, an integrated economizer allows both to function together. Constant volume systems rarely include integrated economizer operation, but again not with the combination of ventilation fan reduction and/or heating/cooling fan reduction.
The most common style of changeover sensor is a “dry bulb” sensor. This is simply the measured sensible temperature of the outside air. A common dry bulb changeover temperature range is 55-60 degrees Fahrenheit. Dry bulb sensors are most prevalent because they are the lowest cost solution.
In areas where humidity is a particular concern, “enthalpy” changeover is often preferred. Humidity contains heat that cannot be measured by a dry bulb temperature probe or sensor. For this purpose, an enthalpy sensor is required. Enthalpy is a measurement of the “total” heat in the air and is measured in BTUs/lb of air. By using enthalpy control, the system more accurately assesses the suitability of outside air for free cooling. Optimizing the use of outside air for free cooling ultimately reduces the energy use of these HVAC systems.
Regardless of the changeover sensor used, any static changeover setpoint will fail to achieve the highest optimized condition when it comes to free cooling. While enthalpy does allow for better control of an economizer there are additional strategies available for improving energy usage. One of these strategies is known as “differential economizer control” and involves the use of two sensors. One sensor references the condition of the outside air and the other sensor references the condition of the return air from the space. Differential economizer control compares both sensors and decides if it is more advantageous to mechanically cool return air from the space or outside air. This strategy results in improved energy efficiency though is not typically used in constant volume systems due to cost considerations.
It is reported that approximately half of all U.S. commercial floor space is cooled by self-contained, packaged air-conditioning units, most of which sit on rooftops. The energy saving potential from optimizing the economizer, ventilation, and fan operation of these HVAC systems is enormous.